Mark is spot on. Regarding the question of shooting in 4K or HD for actual HD production. To me it makes absolutely no sense to shoot in HD as opposed to 4K. Why would you not want to have a permanent archived file for later playback in 4K? Once you've shot whatever you've shot and wherever you shot it, it's gone. So shoot it in the best quality for 4K now or 4K later...even years later.
As for the other rationale, the downscaled HD simply looks great on an HD screen...better than probably anything you've ever seen from an HD camera. To be honest, I haven't even attempted to shoot in the HD mode, but according to Sony, it will look better watching in downcoverted HD as opposed to shooting in HD. It makes all the sense in the world.
Now for editing, many editing programs will automatically downrez the 4K file to HD if you set up your project as a regular HD project. So in many cases it's no more than dropping your file on the timeline.